When you run in a group...especially a larger group...the energy of the group is usually dissipated because some people run faster than others. Consequently, the group fractures into little sub-groups. A big group run turns into a lot of little group runs. It's my feeling that it loses a little something.
If you use walk breaks...you can make it so everyone uses the walk break to return to each other...rather than forge ahead. In other words...during the walk break...the people out ahead turn around and walk back to the folks running slower. You can look at the person in the back as home base. They are, in a sense, leading the workout. It's nice.
As long as the faster folks don't feel the need to put great amount of distance on the slower folks...and the slower folks don't have an ego problem...it works great. Everybody ends up running the pace that suits them and yet you still stick together.
I've done it many times...sometimes when I'm the fast guy and sometimes when I'm the slow guy...and I feel it keeps everything much more cohesive and you derive the intangible benefits of working as a group.
Remember to walk with fast cadence...and carry your arms in the running position. It's not a rest...just a temporary switch to fast walking.
Imagine approaching the last 10 minutes of a 2 1/2 hour marathon-prep effort on the hilly Service Rd with 20 other people right alongside...as opposed to being by yourself or maybe with one other person who happens to be at your level. Maybe give it a try.